1. Field of the Invention
The present invention relates to semiconductor manufacturing and, more particularly, to a method for forming a dielectric layer of a semiconductor device by atomic layer deposition (ALD) and a capacitor using the same.
2. Description of the Related Art
As semiconductor memory devices increase in memory cell density, there is a continuing challenge to maintain sufficiently high storage capacitance despite decreasing cell area. Thus, much effort is spent on obtaining a sufficient capacitance on a limited area of cell. There have been various attempts to achieve this goal. One such attempt is to employ a capacitor dielectric layer having a high-dielectric constant. As another example, hemispherical grain (HSG) polysilicon electrodes are introduced to increase an available area of the cell area.
In line with these efforts, a metal oxide film of aluminum or tantalum is used as a dielectric material, replacing conventional silicon oxide film or nitride film. However, such high dielectric constant materials have drawbacks such as poor leakage current characteristics.
U.S. Pat. No. 5,923,056 issued to Lee, Woo-Hyeoung, et al. discloses metal oxides for use in a semiconductor device. Here, aluminum oxide (Al2O3) is doped with dopants of Group IV materials, for example, Zr, Si, Ti, and the like at about 0.1 percent to about 30 percent by weight of aluminum oxide in order to improve the leakage current characteristics. In addition, a stacked layer of doped aluminum oxide and undoped aluminum oxide is disclosed. The doped aluminum oxide films are formed on the substrate using a reactive sputtering. An aluminum target with 1 percent by weight of silicon distributed uniformly in the target is used to form the silicon-doped films. An aluminum target with 0.5 weight percent of zirconium distributed uniformly in the target is used to form the zirconium-doped films.
Furthermore, the fabrication of a multi-layer structure by an atomic layer deposition (hereinafter, referred as ALD) process is also suggested. The conventional ALD provides a thin film by supplying reactants in pulses, separated from each other by a purge gas, i.e., alternatively pulsing a first precursor gas and a second precursor gas separated from the first precursor gas into the region of the substrate surface. Between precursor pulses the process region is exhausted and a pulse of purge gas is injected. The ALD process offers several advantages over a conventional CVD (chemical vapor deposition) process. According to the conventional ALD process, the composition of a thin film can be controlled more precisely, and contaminant particles can be reduced. Furthermore, the step coverage of layer in ALD process is better as compared with the step coverage of the conventional CVD layer.
To further describe the concept of ALD, FIG. 1 illustrates a gaseous reactant AXn (g) supplied to the surface of a wafer 10 to form a thin film thereon in a reaction chamber. The portion of the supplied reactant AXn (g) is chemically adsorbed (xe2x80x9cchemisorptionxe2x80x9d) onto the surface of the wafer 10 in a solid state. Another portion of the supplied reactant is also physically adsorbed (xe2x80x9cphysisorptionxe2x80x9d) onto the chemisorbed reactant (xe2x80x9cphysisorptionxe2x80x9d). The chemisorption occurs most at the wafer surface while the physisorption occurs primarily on the chemisorbed reactant.
As shown in FIG. 2, when the physically adsorbed (xe2x80x9cphysisorbedxe2x80x9d) reactant AXn(g) is removed by purging, the chemically adsorbed (xe2x80x9cchemisorbedxe2x80x9d) AXn(s) in a solid state remains on the surface of the wafer 10. Herein, Xn indicates a chemical ligand including n radicals.
As shown in FIG. 3, when vaporized H2O is introduced to the surface of the chemisorbed AXn(s), A is oxidized to produce AO on the wafer surface and Xn radical is reacted with H radical to give HXn (g), which can be removed from the wafer surface by purging.
As shown in FIG. 4, the residual impurities such as chemical ligands are removed from the wafer surface by purging. Thus, the oxide film, which consists of an atomic layer of AO(s) in a solid state, is formed.
Referring to FIG. 5, another reactant BYn (g) is introduced or injected into the reaction chamber in the same method as described above. The reactant BYn (g) is chemisorbed onto the surface of the oxide of AO(s). Then, an oxide film including BO(s) is formed on the oxide film of AO(s) in the form of an atomic layer through purging, oxidizing and purging processes.
Accordingly, two types of the oxide films are formed on the wafer surface alternately and repeatedly, thereby forming a dielectric layer comprising plural atomic layers as shown in FIG. 5.
In the conventional ALD method, however, as gaseous reactants A and B are separately introduced into the reaction chamber to form an atomic layer, an overall thin film deposition process is very complicated. In addition, by-products are generated due to the chemical reaction of the reaction gases A and B, and the concentration of the impurity in the oxide film increases.
Accordingly, a need arises for an improved ALD method for forming an atomic layer without such problems.
The present invention contemplates a method for forming a dielectric layer by ALD. According to one embodiment of the present invention, a semiconductor wafer is loaded into a reaction chamber. A source gas mixture containing at least two mixed chemical reactants, e.g., first and second metal compound gases, is introduced into the reaction chamber, thereby chemically adsorbing a portion of the source gas mixture onto a surface of the wafer. The chamber is purged or pumped to remove physisorbed reactants therefrom. The source gas mixture chemically adsorbed on the surface of the wafer is oxidized to form an atomic layer thereon.
According to another embodiment of the present invention, the first metal compound gas is aluminum compound gas, such as TMA (Tri Methyl Aluminum) or TEA (Tri Ethyl Aluminum), and the second metal compound gas is titanium compound gas, such as TiCl4, TDMAT (Tetra Dimethylamino Titanium) or TDEAT (Tetra diethylamino Titanium). The oxidation gas used in the present invention comprises one selected from the group consisting of H2O, O3, O2 plasma, N2O and mixtures thereof.
A dielectric layer according to another embodiment of the present invention is formed as a multi-layer structure of atomic layers by forming the first atomic layer and the second atomic layer on the surface of the wafer repeatedly and alternately. The dielectric constant of the dielectric layer comprising a multi-layer structure of atomic layers can be adjusted by varying a ratio of the first metal compound gas to the second metal compound gas.